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<span id="GOMP_005fCPU_005fAFFINITY-_002d_002d-Bind-threads-to-specific-CPUs"></span><h3 class="section">4.18 <code>GOMP_CPU_AFFINITY</code> &ndash; Bind threads to specific CPUs</h3>
<span id="index-Environment-Variable-17"></span>
<dl compact="compact">
<dt><em>Description</em>:</dt>
<dd><p>Binds threads to specific CPUs.  The variable should contain a space-separated
or comma-separated list of CPUs.  This list may contain different kinds of 
entries: either single CPU numbers in any order, a range of CPUs (M-N) 
or a range with some stride (M-N:S).  CPU numbers are zero based.  For example,
<code>GOMP_CPU_AFFINITY=&quot;0 3 1-2 4-15:2&quot;</code> will bind the initial thread
to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to 
CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12,
and 14 respectively and then start assigning back from the beginning of
the list.  <code>GOMP_CPU_AFFINITY=0</code> binds all threads to CPU 0.
</p>
<p>There is no libgomp library routine to determine whether a CPU affinity
specification is in effect.  As a workaround, language-specific library 
functions, e.g., <code>getenv</code> in C or <code>GET_ENVIRONMENT_VARIABLE</code> in 
Fortran, may be used to query the setting of the <code>GOMP_CPU_AFFINITY</code> 
environment variable.  A defined CPU affinity on startup cannot be changed 
or disabled during the runtime of the application.
</p>
<p>If both <code>GOMP_CPU_AFFINITY</code> and <code>OMP_PROC_BIND</code> are set,
<code>OMP_PROC_BIND</code> has a higher precedence.  If neither has been set and
<code>OMP_PROC_BIND</code> is unset, or when <code>OMP_PROC_BIND</code> is set to
<code>FALSE</code>, the host system will handle the assignment of threads to CPUs.
</p>
</dd>
<dt><em>See also</em>:</dt>
<dd><p><a href="OMP_005fPLACES.html">OMP_PLACES</a>, <a href="OMP_005fPROC_005fBIND.html">OMP_PROC_BIND</a>
</p></dd>
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